School lunches are really gross. When I went to school the lunch ladies actually cooked. The new schools' cafeterias are mainly freezers with defrosting capabilities for kitchens, we used to have actual kitchens. And the lunches are sub contracted out. It's crap and my kids won't eat it.
When I was a kid, all the kids complained about the food. But looking back, it really was fairly decent, and it was nutritious and well balanced. We didn't have very many fat kids when I was a kid, at least that I can remember. There were a couple, but they were the exception.
For the most part though, outside of school, I think you will find that the lower the income, the greater the chance of kids being fat. But it's not because poor people can't afford to purchase the right foods; they just choose to eat crap. I spend around $500 per month on food for myself and my two teenage boys. We eat crap for snacks quite a bit, but we also eat a decent dinner every night. For lunch, my boys will normally eat a sandwich, a piece of fruit, some chips and a cookie. And they only drink milk at mealtime, whether it be breakfast, lunch, or dinner.
As an example of what we eat for dinner, tonight we had steelhead trout with scalloped potatoes, and peas. The meal cost about $10.00 and took 30 minutes to prepare. It's really not that difficult.
Besides the food we eat, the lack of physical activity among our kids is another huge problem. My kids are in sports at school. Both run cross-country, wrestle, and are on the track team in spring. It really doesn't matter what activity they do so long as they are doing something consistently. Sitting in front of the TV playing X-box doesn't count. The bottom line is that it is the parent's responsibility. When you cook dinner and the kid doesn't like what you cooked, you have a choice; make him/her eat it or starve, or let him open a bag of chips and have some ice cream instead.
Too many parents believe it's not good to force kids to eat things they don't like. Well, my kids eat what I put in front of them, and because of that, they now like, or at least will tolerate, just about anything I feed them. They eat every type of meat, fish, and seafood you can imagine. Actually, if we do go out to eat, that can become expensive because they've learned to appreciate things like crab legs, lobster, and all kinds of other good things.